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191.
The infrared and laser-Raman spectra of crystalline Mg3(VO4)2 are reported and a factor group analysis for the full lattice is presented to support the discussion. The 1,000 to 600 cm?1 region of the spectrum is essentially related to internal (stretching) modes of the greatly distorted VO4 tetrahedra, whereas the bands in the low frequency region must be assigned to complex vibrations originating in the interaction of the bending vibrations of the tetrahedral groups with the external modes. 相似文献
192.
Dakoji S Li D Agnihotri G Zhou HQ Liu HW 《Journal of the American Chemical Society》2001,123(40):9749-9759
The inhibitory properties of (methylenecyclopropyl)formyl-CoA (MCPF-CoA), a metabolite derived from a natural amino acid, (methylenecyclopropyl)glycine, against bovine liver enoyl-CoA hydratase (ECH) were characterized. We have previously demonstrated that MCPF-CoA specifically targets ECHs, which catalyze the reversible hydration of alpha,beta-unsaturated enoyl-CoA substrates to the corresponding beta-hydroxyacyl-CoA products. Here, we synthesized (R)- and (S)-diastereomers of MCPF-CoA to examine the stereoselectivity of this inactivation. Both compounds were shown to be competent inhibitors for bovine liver ECH with nearly identical second-order inactivation rate constants (k(inact)/K(I)) and partition ratios (k(cat)/k(inact)), indicating that the inactivation is nonstereospecific with respect to ring cleavage. The inhibitor, upon incubation with bovine liver ECH, labels a tryptic peptide, ALGGGXEL, near the active site of the protein, where X is the amino acid that is covalently modified. Cloning and sequence analysis of bovine liver ECH gene revealed the identity of the amino acid residue entrapped by MCPF-CoA as Cys-114 (mature sequence numbering). On the basis of gHMQC (gradient heteronuclear multiple quantum coherence) analysis with [3-(13)C]-labeled MCPF-CoA, the ring cleavage is most likely induced by the nucleophilic attack at the terminal carbon of the exomethylene group (C(2)'). We propose a plausible inactivation mechanism that involves relief of ring strain and is consistent with examples found in the literature. In addition, these studies provide important clues for future design of more efficient and selective inhibitors to control and/or regulate fatty acid metabolism. 相似文献
193.
Zusammenfassung Es wurde die Absorption und Dispersion verschiedener Polyäther im Mikrowellengebiet und im fernen IR untersucht. Mit wachsender Sauerstoffzahl in der Kette werden die Torsions- und Biegeschwingungen wachsend IR-aktiv. Ihre Frequenzlagen ändern sich mit dem zunehmend polaren Charakter der Struktur. Die Analogie zwischen der Verschiebung der Schwingungsfrequenzen und dem Verlauf der Schmelztemperaturen wird diskutiert.Mit zunehmend polarem Charakter steigen der Temperaturkoeffizient der Molpolarisation und die Differenz (–n
2) der Dielektrizitätskonstanten im Mikrowellengebiet und im optischen Bereich stark an. Ein Vergleich mit der Höhe der Dispersionsstufen im fernen IR und deren Temperaturabhängigkeit ergab, daß diese zusätzliche Polarisation nicht allein durch die IR-aktiven Banden bedingt ist. Sie muß gekoppelt sein mit dem ebenfalls stark anwachsenden Untergrundverlust im gesamten mm- und sub-mm-Gebiet.
Mit 10 Abbildungen in 14 Einzeldarstellungen und 2 Tabellen 相似文献
Summary Absorption and dispersion of several polyethers were measured in the microwave region and in the far infrared. The torsional vibrations become increasingly iractive with increasing number of oxygens in the chain. Their frequencies shift, too, in a manner analogues to the variation of the melting temperatures.The temperature coefficient of the mol polarization at 32 GHz and the difference (–n 2) between microwave and optical dielectric constant rise rapidly with increasing polar character of the structure. It was tried to correlate (–n 2) with the overall absorption of the skeletal vibrations. Height and temperature dependence of the far ir dispersion show, however, that the larger part of this additional polarization cannot be accounted for by the resonance absorption bands. It must be correlated with the background loss which extends over the whole mm and sub-mm range and also increases rapidly with increasingly polar structure.
Mit 10 Abbildungen in 14 Einzeldarstellungen und 2 Tabellen 相似文献
194.
195.
Summary Swelling effects of carbon tetrachloride on the mechanical relaxations in the crystal mat and the melt-crystallized samples of high density polyethylene were studied using a torsion pendulum. A very strong-relaxation peak appears by swelling at the low temperature side of the original-relaxation for the crystal mat. It was shown that this strong-relaxation originates from the slippage between the highly swollen lamellae. For the melt-crystallized sample, the-relaxation peak is decreased by swelling, but its temperature is unchanged. This behavior suggests that the-relaxation in the melt-crystallized sample originates mainly from deformation process of intermosaic block region. The swelling effect on-relaxation was also discussed.
Zusammenfassung Quellungseffekte von Tetrachlorkohlenstoff auf das mechanische Relaxationsverhalten in Einkristall-Matten und schmelzkristallisierten Proben von linearem Polyäthylen wurden mit den Torsionspendeln untersucht. Ein sehr starkes-Maximum tritt durch die Quellung bei tieferen Temperaturen als der der-Relaxation in den nicht gequollenen Einkristall-Matten auf. Es wird gezeigt, daß diese starke-Relaxation auf das Gleiten der ausreichend gequollenen Lamellen zurückzuführen ist. Bei schmelzkristallisierten Proben wird das-Maximum durch die Quellung erniedrigt, seine Temperaturlage wird jedoch nicht verändert. Dies zeigt, daß die-Relaxation in der schmelzkristallisierten Probe hauptsäcblich auf Deformations-Prozessen in den Blöcken beruht. Auch die Quellungseffekte auf die-Relaxationen wurden diskutiert.相似文献
196.
Dr. E. L. Mehler 《Theoretical chemistry accounts》1974,35(1):17-32
A new method is proposed for calculating correlation effects in atomic and molecular systems. The basis of the method is the
formulation of a set of partial configuration expansions which yield directly variational orbital correlation corrections
which are appropriately summed in order to obtain an estimate of the total correlation energy. This method is applied to the
ground state of boron hydride and its cation at the equilibrium distance of BH. The results of the method are compared in
detail with independent electron pair results and second order CI results. It is further shown that multiple substitutions
are approximately accounted for in this method and the extent to which they are included is compared with other approximations.
Finally, three methods of increasing accuracy, aimed at reducing the necessary computational effort, are given for determining
the vertical ionization potential. The most economical method yields an IP of 9.70 eV or 0.03 eV less than the experimental
IP. Completion of the basis is estimated to improve this value to 9.77 eV. 相似文献
197.
Xe(OTeF5)2 reacts with Sb(OTeF5)3 under the formation of [Xe2(OTeF5)3]+[Sb(OTeF5)6]-. From SO2ClF solution a yellow solvate [F5TeOXe]+·SO2ClF· [Sb(OTeF5)6]- is formed with the crystal data: a = 1028.1(1), b = 1040.9(1), c = 1780.2(3) pm, α = 98.07(1), β = 97.68(1), γ = 105.82(1)°, space group . The O-Xe···O fragment is essentially linear (176.1(2)°), and the two Xe-O distances are quite different 197.1(4) and 242.6(4) pm. 相似文献
198.
199.
Nan Zhou 《Mikrochimica acta》1991,105(4-6):159-162
Proposed in this paper is an improved method for removal of tin as the matrix element by volatilization in an anhydrous medium as mixed halides of different composition. Any one of them boils at a lower temperature than SnBr4. This ternary system enhances the eventual recombination of temporarily dissociated species to volatile ones, thus making the process quicker and more efficient. The presence of 2 mg of Pb(II) suffices to prevent Bi(III) from loss due to volatilization. 相似文献
200.
Zhixian Zhou Yuzhong Wang Jingchao Tao Yaoting Fan Yangjie Wu 《Journal of inclusion phenomena and macrocyclic chemistry》1998,32(1):69-80
A novel anion-selective electrode has been prepared by usingtetraazaporphyrinogen as the electroactive component and o-nitrophenyloctylether as the plasticizer. The electrode exhibits almost Nernstian responsecharacteristics for Pic-, ReO
4
-
,SCN-, ClO
4
-
and TPB-. The linearresponse ranges towards the above-mentioned anions are 10-6 10-2, 10-5 10-2,10-5 10-2, 10-5 10-1, and 10-5 10-2mol/Land the corresponding slopes are -56.8, -57.1, -56.3, -56.1, and -59.9mV/decade with correlation coefficients of -0.99978, -0.99987, -0.99999,-0.99998, and -0.99998, respectively. The electrode shows an anti-Hofmeisterselectivity sequence: Pic- > SCN- >ReO
4
-
> ClO
4
-
> I- >Br- > BF
4
-
> Sal->NO
3
-
> Cl-. The unusual responsemechanism of the novel anion-selective electrode was investigated byexperimental observations and calculation with the MNDO method. Theelectrode was used as a TPB- and a Pic-sensitiveelectrode, respectively, and applied to the assay of levamisolehydrochloride tablets by potentiometric titration and Gran's methods. Theresults obtained are in excellent agreement with that determined by thestandard pharmacopoeia method based on nonaqueous titration. 相似文献